Every Wednesday after 2:00 pm (GMT), the market freezes in anticipation of the release of the EIA's statistics on commercial crude oil reserves. Increased reserves (relative to the forecast data) means that either too much oil was produced or consumption was lower. But, all things being equal, this often leads to lower oil prices.
In contrast, lower reserves means increased consumption or decreased production, which is a bullish factor. Markets are also watching alternative statistics on reserves from the API, which are usually released the day before, on Tuesday.